1. Technical Field
The invention relates to an aspheric-lens surface-decenter measuring method and apparatus for measuring surface decenter of an aspheric lens for use in various optical devices such as a digital camera and an optical sensor.
2. Description of the Related Art
Recently, most aspheric lenses are formed by molding. However, due to a relative positional misalignment between molds, a surface decenter (a relative positional misalignment between respective axes of two lens surfaces of an aspheric lens) or a surface tilt (a relative inclination misalignment between respective axes of two lens surfaces of an aspheric lens) may occur in the molded aspheric lenses. It is difficult to completely eliminate such a surface decenter or a surface tilt because of structural mechanism of the molds. However, since the surface decenter and the surface tilt are factors that increase aberrations of the molded aspheric lens, it is preferable to correct the molds so as to decrease the aberrations. For this reason, it is important to measure how much the surface decenter and the surface tilt there occur.
Various decentration measuring tools for measuring surface decenter (decentration) of an aspheric lens have been practically used. Moreover, a method described in Japanese Patent No. 3127003 is known as a method for measuring surface decenter and surface tilt of an aspheric lens. This method measures surface decenter and surface tilt of an aspheric lens having a flange-like projecting portion provided to be perpendicular to an optical axis of the lens, using an autocollimator.
Also, aberrations of an aspheric lens can be obtained by measuring a transmissive wavefront using an interferometer. For example, such a method is known that wavefront aberration obtained by measuring transmissive wavefront is expanded using a power series such as the Zernike polynomials to calculate the Seidel's five aberrations (see Japanese Patent No. 2951366).
Recently, demand for an aspheric lens having a high numeric aperture (NA) is growing. In such a high-NA aspheric lens, surface decenter of several micrometers (μm) and surface tilt of several tens of seconds are acknowledges as a problem, which was not taken seriously in the past.
Meanwhile, there is also a demand for measuring the surface decenter and the surface tilt in a separate manner with high precision. Depending on a type of an optical system, the optical system may be highly sensitive to the surface decenter of the aspheric lens used but less sensitive to the surface tilt thereof. Therefore, if it is possible to measure the surface decenter and the surface tilt separately, it becomes possible to determine as to whether the aspheric lens can be used in the respective optical systems. Moreover, when the aspheric lens is actually used, it is also possible to take measures to correct the surface decenter and the surface tilt.
However, the presently available decentration measuring tools cannot measure the surface decenter and the surface tilt separately and have relatively high measurement error on the order of about 2 micrometers. In addition, the method described in Japanese Patent No. 3127003 has difficulties in measuring the surface decenter at a highly precise level conforming to the recent demands.